Vapor degreasing is a well-known, simple and effective method of cleaning workpieces. Chemical-solvent vapor degreasing systems typically comprise an open-topped tank designed to contain a bath of chemical degreasing or cleaning solvent. Heating coils are mounted on or adjacent to the bottom of the tank and are immersed within the solvent. The heating coils boil the solvent and generate vapors which condense on the workpieces, thereby dissolving oil and grease and removing other contaminants from the workpieces.
To prevent the environmentally-deleterious, chemical degreasing solvent from escaping the tank through the open top, it is known to provide a primary solvent condenser positioned intermediate the tank, and a freeboard chiller or secondary solvent condenser positioned above the primary solvent condensor proximate the open top of the tank. The primary and secondary condensors typically comprise a series of low-temperature, finned refrigeration coils located on the inner walls of the tank. Low-temperature refrigerant flows through the coils to create primary and secondary barriers which inhibit the loss of evaporated degreasing solvent.
The temperature of the refrigerant circulating through the freeboard chiller is typically less than 0.degree. F. Therefore, the condensing coils of the freeboard chiller must be defrosted periodically to maintain their efficiency. To defrost the freeboard chiller, the flow of low-temperature refrigerant to the freeboard chiller must be temporarily interrupted by turning off the refrigeration unit. If a single refrigeration unit is used to cool both the freeboard chiller and the primary solvent condensor, the degreasing operation must be shut down during the defrosting cycle since both condensers will be deactivated when the refrigeration unit is turned off. If the degreasing operation is not shut down, the vaporized degreasing solvent will freely escape the tank.
To eliminate this "down-time", it is known to provide two separate refrigeration units for cooling the freeboard chiller and the primary solvent condensor. With two separate refrigeration units, the freeboard chiller can be defrosted without deactivating the primary solvent condensor. Therefore, the degreasing operation can be continued during the defrost cycle, albeit with a small loss of vaporized solvent from the top of the tank. However, a degreasing system having dual refrigeration units is more costly and is less efficient than a system having a single refrigeration unit.
Therefore, it would be desirable to provide a refrigerated vapor degreasing system which requires only one refrigeration unit for cooling both the freeboard chiller and the primary solvent condenser.
Additionally, it would be desirable to provide a degreasing system with single refrigeration unit wherein the freeboard chiller can be defrosted while still maintaining the flow of low-temperature refrigerant through the primary solvent condenser.